The present invention relates to a method for welding comb-shaped sealing strips to plate heat exchangers for gasses guided in counter-flow, cross counter-flow or transverse flow, especially of cold clean gasses exiting a desulfurization device and of flue gasses for reheating these clean gasses resulting from the operation of a power plant fueled with fossil fuels, especially refuse incinerators.
Such plate heat exchangers are comprised of a plurality of pairs of plates, preferably made of highly corrosion-resistant material. The pairs, in turn, are connected to form a plate stack whereby, for separating the gas streams participating in the heat exchanging process, comb-shaped sealing strips are arranged at the edges as well as optionally at the center portion of the plate stacks.
Especially when highly corrosion-resistant material, for example, chromium molybdenum steels that have a high nickel contents, such as Hasteloy, is used as the material for the individual plates and the sealing strips, it is difficult to produce gas-tight welding seams of high corrosion resistance. It is also problematic that parts must be welded together that have a different wall thickness. While the individual plates have a sheet thickness of, for example, 0.37 mm, the wall thickness of the sealing strips is, for example, approximately 2.6 mm. Further complicating matters is the fact that the edges of these thin sheet metals must be welded to the wide side, i.e., the large surface of the sealing strip.
In the past the welding process of welding the comb-shaped sealing strips to the individual plates combined to plate pairs of the heat exchanger has been carried out by using a tungsten inert gas welding process whereby very thick welding seams are produced. Upon examining the welding seams produced in this manner, it has been proven time and time again that these welding seams are not gas-tight due to micropores. These micropores are caused by the extreme heat introduced during welding. Furthermore, it has been shown that the corrosion resistance of such welding seams is not sufficient with respect to the desired specifications even though highly corrosion resistant materials have been used not only for producing the individual plates and sealing strips, but also as the welding material. Tests have shown that the initially highly corrosion resistant welding material is subjected to a material change due to the extreme heat produced during welding and due to the burn-up during welding. This material change makes the starting material substantially more corrosion-sensitive.
It is therefore an object of the present invention to improve the aforementioned method of welding of comb-shaped sealing strips to plate heat exchangers such that even in a method for mass-production not only gas-tight but also highly corrosion-resistant welding seams can be produced.